This invention relates to a copper base alloy for leads used in an integrated circuit (hereinafter called "IC"), and more particularly to such alloy which possesses satisfactory properties such as mechanical property, thermal resistance, corrosion resistance and thermal conductivity (electric conductivity) which are required of IC leads.
IC leads, which are bonded to an IC tip to serve as external lead wires, are usually made of a material such as copper base alloy. Such material for IC leads is generally required to satisfy the following property requirements:
(a) Sufficient mechanical property. That is, the following two requirements which are contradictory to each other must be fulfilled: While IC leads should neither twist nor bend when stress is applied to them during assemblage or transportation of ICs, they should not break when they are repeatedly bent in connecting them to wiring boards during mounting of ICs onto an electronic appliance. To satisfy the former requirement they should have necessary hardness, while to satisfy the latter requirement, they should have necessary elongation. If they show a Vicker's hardness of 135 -240 kg/mm.sup.2 and an elongation of not less than 6%, they can satisfy the above-mentioned requirements.
(b) High thermal resistance. That is, an ordinary process of manufacturing ICs includes the steps of bonding a semiconductor tip (e.g., silicon tip) to a substrate, connecting IC leads to the semiconductor tip by means of gold wires or aluminum wires and coating said IC leads with silver or tin-lead solder alloy. In these steps, the IC leads are heated up to a temperature of 350.degree.-400.degree. C. On this occasion, the IC leads are softened by the heat, resulting in that their surfaces may have a reduced flatness, and simultaneously they may have a degradation in the mechanical property or hardness of the leads, which was imparted to a strip blank which is to be stamped into leads, during cold rolling thereof due to work hardening. Therefore, IC leads must have a thermal resistance enough to be free of softening even when heated to a temperature of from 350.degree. to 400.degree. C., that is, they should have a softening point of not less than 400.degree. C. If the softening point is 400.degree. C. or higher, that is, the higher the softening point is, the more excellent mechanical property and flatness the IC leads have, which eliminates the risk that the IC leads soften due to irregularities in the assembly working of the ICs during the above-mentioned steps.
(c) Low thermal conductivity. That is, in manufacturing an IC, a semiconductor tip and gold wires (or aluminum wires), etc. are packed in a plastic material, with the foot portions of the IC leads exposed. After this plastic packing, the exposed portions of the IC leads are coated with silver or tin-lead solder alloy. In mounting the IC into an electronic appliance, the IC leads are soldered onto a substrate or the like. In these steps, heat produced by said coating or soldering conducts in the IC leads, which may deteriorate the plastic package and a plastic bond binding together the semiconductor tip and the substrate. The possibility of such deterioration increases in proportion to the thermal conductivity of the IC leads. Therefore, it is desirable that IC leads should have a low thermal conductivity. Thermal conductivity being proportional to electric conductivity, generally the thermal conductivity of IC leads should be less than 60% I. A. C. S. (International Annealed Copper Standard) in terms of electric conductivity. However, if IC leads have a too low electric conductivity, they have a low heat radiation. Therefore, IC leads should have an electric conductivity of from 35 to 55% I. A. C. S.
(d) High corrosion resistance. That is, although silver or tin-lead solder alloy coating as previously referred to can improve the corrosion resistance of IC leads to some extent, corrosion may proceed in the IC leads during a long period of use, which may cause trouble unless the IC leads themselves have some corrosion resistance. The standard value of the corrosion resistance of IC leads should be 0.20 .mu.m in terms of mean surface roughness which is obtained after a 48 hour's salt spray test according to Japanese Industrial Standard (JIS) Z 2371 and which is judged as a passable value. The IC leads should have a corrosion resistance corresponding to a value of mean surface roughness not higher than said standard value.
Alloys for IC leads should therefore have properties satisfying the requirements mentioned in the above paragraphs (a)-(d). That is, it can be concluded that an alloy which has a mechanical property, a thermal resistance, a thermal conductivity and a corrosion resistance satisfying the following values respectively is most suitable for IC leads:
(1) Vicker's hardness: 135-240 kg/mm.sup.2 PA1 (2) Elongation: not less than 6% PA1 (3) Softening point: not lower than 400.degree. C. PA1 (4) Electric conductivity: 35-55% I. A. C. S. PA1 (5) Mean surface roughness after a salt spray test: not higher than 0.20 .mu.m (after a 48 hour's salt spray) PA1 Copper + tin + phosphorus: not less than 99.5% PA1 Lead: not more than 0.05% PA1 Iron: not more than 0.10% PA1 Tin: 1.5-2.0% PA1 Phosphorus: not more than 0.30% PA1 Iron: 0.5-1.5% by weight, PA1 Tin: 0.5-1.5% by weight, PA1 Phosphorus: 0.01-0.35% by weight, and PA1 Copper and inevitable impurities: the balance. PA1 Iron: 0.5-1.5% by weight, PA1 Tin: 0.5-1.5% by weight, PA1 Phosphorus: 0.01-0.35% by weight, and PA1 Copper and inevitable impurities: the balance.
While, alloys conventionally used in the manufacture of IC leads include a copper base alloy No. C 50700 (hereinafter called "CDA Alloy 507") which is in accordance with the standards established by CDA (Copper Development Association Inc.), and which has the following chemical composition:
Although this alloy shows satisfactory values in respect of hardness and elongation and also shows about 35% I. A. C. S. in respect of electric conductivity, it still fails to show satisfactory results in respect of thermal resistance and corrosion resistance.